This invention relates to foil laminating adhesives and more particularly to latex foil laminating adhesives incorporating a carboxylated vinyl acetate copolymer thickener.
Adhesives useful for laminating thin metal foils such as aluminum foil to substrates such as paper and paperboard must possess certain properties which are not found in adhesives generally. In order to form the required thin film of adhesive between the foil and the substrate they must have special viscosity properties; in particular they must be to some extent thixotropic. In addition they must not dry out too rapidly when being applied, with consequent fouling of the laminating machine, and must not form hard deposits on the parts of the laminating machine which cannot be removed with water. These properties are not capable of definition in terms of exact numerical values determined by simple tests such as by measurement of viscosity, but are rather determined in actual practice by the performance of a particular adhesive when actually used in a laminating machine to laminate foil to a substrate. One skilled in the art of foil laminating can readily determine in actual use which adhesives are suitable for foil laminating by examining the performance of an adhesive when used in the laminating machine and by examining the laminated product.
Hitherto only foil laminating adhesives prepared by thickening an adhesive latex with an aqueous solution of casein have provided the required properties. The preferred foil laminating adhesive has been a polychloroprene latex thickened with casein. However, adhesives thickened with casein have certain disadvantages. Such adhesives tend to form lumps or settle in the container during storage. They are subject to viscosity increases with age. Since casein is a natural product it is subject to spoilage and also has an odor which is objectionable in some applications. Finally, since casein is a natural product the price is subject to the fluctuations of the market, hence securing an adequate supply of the material of the right quality and at a stable price can be a problem.
It has now been found that certain carboxylated vinyl acetate copolymers can be used as thickening agents for adhesive latices to produce a foil laminating adhesive having properties comparable to those of the best prior art foil laminating adhesives and with certain advantages over the prior art adhesives.